The invention relates to a brake system and a vehicle comprising a brake system according to the preambles of the independent claims.
Mobile work machines are vehicles designed for and used in rough off-road surroundings where trucks or passenger cars are inoperative and would be damaged when exposed to these rough conditions. During operation particularly in rough surroundings, the speed of the work machine has to be controlled reliably, particularly at low speed of the work machine, e.g. when operating downhill with a heavy load.
In order to control the speed of the work machine particularly at low speed it is known in the art to use the service brakes, which, however, requires an adequate dimension of the service brakes. Further, it is known to provide an additional hydraulic brake, a so called retarder, which can be e.g. an additional hydraulic pump in the powertrain of an articulated hauler.
U.S. Pat. No. 5,507,360 A1 discloses a hydraulic system for a work machine with a pump which can selectively provide hydraulic fluid to a steering system or a dynamic braking system. A selector valve which is responsive to the pressure in the steering system directs the flow from the pump to the steering system when the pressure drops below a predetermined level. The operator may selectively direct flow from the pump to the dynamic braking system when dynamic braking is needed.
It is desirable to provide a robust and economically priced braking system which fulfils particularly the requirements of work machines. It is also desirable provide an improved vehicle comprising a braking system.
According to a first aspect of the invention, a brake system for a vehicle is proposed, comprising at least one hydraulic pump, wherein the at least one hydraulic pump is connected to an input fluid conduit and an output fluid conduit. A flow resistance unit is provided in the output fluid conduit of the at least one hydraulic pump for being selectively coupled into the output fluid conduit when brake torque is requested.
Favourably, one or more hydraulic, pumps which are already available in the vehicle can be employed and used as a retarder. The retarder effect can be established by transmitting the kinetic energy, i.e. speed of a power source of the vehicle and/or vehicle speed, to heat the hydraulic fluid when there is a brake demand. When a brake torque is requested, the flow resistance unit is brought into operational connection with the output fluid conduit of the at least one pump.
A smaller number of parts is necessary and valuable construction space in the vehicle can be saved as well as costs. The brake system is particularly robust. The reaction time can be very short. It is not necessary to fill the retarder with hydraulic fluid before operation and starting the brake effect. The at least one pump pumps hydraulic fluid, e.g. hydraulic oil, against a low pressure and is already filled up.
Favourably, a power-take-off unit adapted for power-take-off from a vehicle powertrain and that the at least one hydraulic pump can be coupled to the power-take-off unit. Preferably, the power-take-off unit can be arranged between the engine and a main transmission of the vehicle.
According to a preferred embodiment at least one service brake can be provided for performing a primary brake function and the at least one hydraulic pump and the flow resistance unit can form a secondary brake function. Thus, the retarder torque can be controlled by controlling the pressure against which the at least one pump is working.
Generally, the service brake can be a wet or a dry brake. In a wet brake, typically disks which can act on a ground engagement element such as a wheel are arranged in a closed environment filled with a liquid such as oil. In a dry brake, a brake caliper and brake pads acting on a ground engagement element are surrounded by air.
Advantageously, the secondary brake function can be controllable according to operation parameters of the primary brake function.
Favourably, a cooler unit can be arranged downstream of the flow resistance unit.
The cooler unit can be connected to a conventional cooling system of the vehicle.
The kinetic energy is transformed into heat by pumping the hydraulic fluid against the flow resistance generated by the flow resistance unit in the output fluid conduit. The heated hydraulic fluid can be cooled down so that the hydraulic fluid is available at normal operation temperatures for braking as well as for operating hydraulic devices in the vehicle.
According to an embodiment of the invention, the at least one hydraulic pump can be adapted for being selectively coupled to at least one hydraulic cylinder.
Favourably, already available components can be used and can be simply coupled to the flow resistance unit if their action is required. The at least one hydraulic pump can be selectively coupled to at least one hydraulic cylinder when brake torque not is requested. Favourably, the at least one hydraulic pump provides hydraulic fluid for operation of hydraulic cylinders, e.g. for steering the vehicle, for tilting a tiltable platform and the like.
A controllable valve can be provided for switching a fluid flow the at least one pump between different fluid conduits. Favourably, the controllable valve can be embodied as a shift valve, particularly as a 5/2-way valve comprising 5 ports and 2 positions. Preferably, the controllable valve can be arranged in the fluid conduit between the at least one pump and the flow resistance unit.
The flow resistance unit can be a pressure regulating valve. The flow resistance unit can be a throttle. It is also possible to provide a combination of a pressure regulating valve and a throttle. The retarder torque can be controlled by controlling the pressure, i.e. the flow resistance, induced by the flow resistance unit.
According to another aspect of the invention, a vehicle is proposed, comprising a powertrain and a brake system according to anyone of the preceding features. The retarder effect caused by at least one hydraulic pump can be established by transmitting the kinetic energy, i.e. speed of the power source of the vehicle and/or vehicle speed, to heat hydraulic fluid when there is a brake demand.
Preferably, the powertrain can comprise a power source and at least one hydraulic pump which can be coupled to a power-take-off unit arranged downstream of the power source. The power source can comprise an internal combustion engine, an electric motor, any other device which can drive a powertrain of a vehicle or any combination thereof. The at least one pump can be directly driven by the power source of the vehicle.
The power-take-off unit can be arranged between the power source and a main transmission of the vehicle.
Preferably, at least one hydraulic cylinder can be used for at least one of steering the vehicle or tilting a tillable platform of the vehicle.
The invention can be applied to wheel-borne vehicles, track-borne vehicles and vehicles running on rails. Primarily wheel-borne vehicles are intended. The invention is particularly useful for work machines, such as frame-steered articulated haulers, wheel loaders, excavators etc. The invention is particularly applicable in vehicles with a multitude of driven axles and will below be described for a frame-steered articulated hauler for the purpose of exemplification. The invention can also be applied to passenger cars, trucks, buses and other road vehicles but is primarily intended for vehicles used off-road in rough conditions. A fundamental problem for all work machines with drive at a number of ground contact points is to control the vehicle safely at low speed, particularly when high load is carried or moved by the vehicle.